Modern telecommunication applications have resulted in substantial increases in the need for additional bandwidth. For example, in the area of wired communications, there is a need to simultaneously support voice, video, and data applications at low BER (Bit Error Rates) using new high-speed modem designs for twisted pairs. At signaling rates better than 10 Mbits/s performance bounds generally exceed a BER of 10−6. When the conventional Pulse Amplitude Modulation (PAM) technique is used, the baseband communication signal is represented by a series of modulated pulses whose amplitude levels are determined by the symbol to be transmitted. For example, with 16-QAM (Quadrature Amplitude Modulation), typical symbol amplitudes of ±1 and ±3 are utilized in each quadrature channel. For digital communication systems, efficient used of bandwidth is crucial when dealing with time-dispersive channel, as is common with wireless systems. In these types of systems, whenever there is distortion of the signals due to preceding or following pulses, normally referred to as pre-cursors and post-cursors, respectively, the amplitude of the desired pulse is affected due to superimposition of the overlapping pulses. This phenomenon is known as intersymbol interference, and is an impediment to high-speed data transmission, especially in systems that are constrained by limited bandwidth.
One way to minimize the effects of intersymbol interference is to use an equalizer. Fixed equalizers are designed to be effectively operated between an upper and lower bound between which the channel is expected to deviate. Whenever these limits are exceeded, the equalizer ceases to operate effectively. Hence, there has to be a certain degree of precision when channel equalization is employed, and fixed equalizers are implemented. There are adaptive equalizers (i.e., continuous) that track dynamic channel dispersion and make continuous adjustments to compensate for such intersymbol interference. This provides some improvement in performance over the fixed equalizer.
Incorporation of the equalizer into some communication systems does not come without penalty. In wireless systems, for instance, insertion loss becomes a critical factor if the equalizer is present and the associated impairment does not occur. The main purpose of the equalizer implementation is to enhance the information bearing capability of the communication system with the design objective of asymptotically approaching the capacity bounds of the transmission channel. Consequently, the use of the equalizer can be regarded as one instance of an array of possibilities that may be implemented to enhance the bit rate of the communication system design.